After a brief breakthrough on Thursday, the Senate gave up on reaching agreement on immigration reform before its two-week recess.

Divisions over this bill run deep through both parties - and members on both sides of the aisle say the Senate needs more time to work though differences.

With massive pro-immigration demonstrations expected nationwide Monday, Republican and Democratic lawmakers are keenly aware of the effect their votes could have in November's elections. An immigration crackdown could mobilize millions of angry Hispanic voters. A more lenient approach could provoke a backlash from voters who oppose any form of amnesty.

On Thursday, senators thought they had crafted a compromise that would fly with both camps. Sens. Mel Martinez (R) of Florida and Chuck Hagel (R) of Nebraska drafted the compromise plan that produced hopes for a breakthrough last week.

The compromise, endorsed by leaders on both sides of the aisle Thursday, would have established a path to legalization for some of at least 11 million immigrants already in the United States illegally. Those who had been in the US more than five years could stay and earn citizenship. Those here between two and five years could file for a temporary work visa, but would have to return to their country of origin to process it. And those here less than two years would have to return home and stand in line with others seeking legal entry.

The revised package still included enhanced border security and a new guest worker program - key elements of a bill passed by the Senate Judiciary Committee.

The compromise stumbled over amendments to the bill. For nearly two weeks, Democrats had blocked all but three of the 400 proposed amendments on the bill. "The amendments were being offered by people who didn't want the bill," argued Democratic leader Harry Reid.

Looking ahead, Democrats worry that if the Senate produced a bill too far from the comprehensive reform that emerged from the Senate Judiciary Committee, that final bill would get watered down in conference negotiations with the GOP-controlled House. "If we went to conference, we would have probably gotten something like the [House] bill back," says Sen. Ken Salazar (D) of Colorado.

Speaking after the vote that derailed the deal, Sen. Arlen Specter, chairman of the Judiciary Committee, blamed Democrats for using Senate procedures to block votes on amendments. "We must take a very, very hard look at abusive practices in the US Senate," he said.

After the Senate returns from a two-week break, the Judiciary Committee will take up consideration of the bill, he added. He said the Judiciary panel could have a bill ready for renewed floor debate in 10 days. Senator Frist, who as majority leader controls the timing of bills on the floor of the Senate, did not commit to a date to reconsider the bill.

Several senators vowed to keep looking for a way through the political thicket.

"I think the next two weeks can be used effectively up here - and for Americans to learn more about what's at stake," said Sen. Hagel. "I am confident we can do something responsible for our country."